The present invention relates to a casting mold for casting workpieces from liquid metal by a single crystal solidification process. The invention finds particular usage in the casting of gas turbine engine blades from superalloy materials.
In known single crystal solidification casting processes a mold is supplied with a molten metal through its upper region and the solidification is controlled such that the solidification front moves gradually vertically through the mold. Typically, a grain selection device is located adjacent to a lower region of the mold and selects a single crystal grain at its discharge outlet. Such known casting methods for single crystal parts are illustrated and described in French patents 1,481,366 and 2,037,187.
A number of techniques are used to achieve a specific crystallographic orientation in the workpiece and to achieve an adequate grain selection. Despite these techniques, problems have existed with the casting of gas turbine engine blades, particularly those blades with platforms. In casting the platform blades, parasitic grains may appear in the blade platform, or the configuration of the platform would preclude certain grain selection. Additionally, the techniques used to cast such platform blades have proven to be extremely lengthy, complex and costly, as well as resulting in excessive numbers of unacceptable parts.
The parasitic grain germination which takes place in the blade platform is caused by a too rapid and premature cooling of the liquid metal in the blade platform. This induces the solidification of the metal from the periphery of the platform independently of the solidification of the remainder of the blade structure. When making a workpiece by the single crystal solidification process, it is necessary to grow a nucleus inside a volume of liquid metal within which no point has reached its liquidis temperature. It has proven to be extremely difficult to control this condition, especially in the case of casting a blade platform wherein the temperature gradient extends normally in a direction generally perpendicular to the larger dimensions (the length and the width) of the blade platform.
The heterogeneous germination of a parasitic grain in the blade platform is enhanced by:
high mold roughness at the time of solidification, which may be aggravated during casting by the reactivity between the high-temperature mold shell and the liquid metal;
a high volume-to-surface ratio;
sharp points or edges.
Under such conditions, the permissible supercooling or solidification delay may not exceed a few degrees. Consequently, as soon as the mold walls and a volume element of the platform reach the liquidis temperature of the cast alloy, there will be a danger of parasitic nucleation and growth of a grain which results in a defective workpiece.
The problems associated with the known techniques are illustrated in FIG. 1, which is a top view of a platform blade 1. The portion 2 has a grain structure resulting from the initial nucleus and is located in the blade airfoil portion 3 as well as a portion 4a of the platform 4. The platform 4 has a length and a width significantly larger than its thickness. The portion 4b of the platform denoted by the hatching lines has a grain structure resulting from a parasitic grain growth.
One proposed solution to this problem was to place a parallel grain input duct between the grain selector outlet and the zone of the blade platform. In this method, the initial grain orientation is reproduced at the cold point of the platform where the grain is growing simultaneously with that of the central portion of the blade. As the growth continues, the two grain portions are bonded together.
However, this technique was extremely difficult to successfully carry out, being both time consuming and specific to each workpiece design. The supply duct adjoining the workpiece required meticulous work in regard to its assembly and removal, which usually was accomplished by forming it in segments. Such segmentation required additional finishing of the workpiece once it had been removed from the mold. Misorientation can occur during grain reattachment and the presence of the supply duct, in some cases, caused grains to recrystallize. All of these drawbacks prevented this technique from resolving the known problems relating to the mass production of such articles.